1. Technical Field
This invention relates to an organic electroluminescent (EL) device, and more particularly, to an inorganic/organic junction structure suitable for use in a device of the type wherein an electric field is applied to a thin film of an organic compound to emit light.
2. Background Art
In general, organic EL devices have a basic configuration including a glass substrate, a transparent electrode of ITO etc., a hole transporting layer of an organic amine compound, an organic light emitting layer of a material exhibiting electronic conductivity and intense light emission such as Alq3, and an electrode of a low work function metal such as MgAg, wherein the layers are stacked on the substrate in the described order.
The device configurations which have been reported thus far have one or more organic compound layers interposed between a hole injecting electrode and an electron injecting electrode. The organic compound layers are typically of two- or three-layer structure.
Included in the two-layer structure are a structure having a hole transporting layer and a light emitting layer formed between the hole injecting electrode and the electron injecting electrode and another structure having a light emitting layer and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Included in the three-layer structure is a structure having a hole transporting layer, a light emitting layer, and an electron transporting layer formed between the hole injecting electrode and the electron injecting electrode. Also known is a one-layer structure wherein a single layer playing all the roles is formed from a polymer or a mixed system.
FIGS. 3 and 4 illustrate typical configurations of organic EL devices.
In FIG. 3, a hole transporting layer 14 and a light emitting layer 15 of organic compounds are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11. In this configuration, the light emitting layer 15 also serves as an electron transporting layer.
In FIG. 4, a hole transporting layer 14, a light emitting layer 15, and an electron transporting layer 16 of organic compounds are formed between a hole injecting electrode 12 and an electron injecting electrode 13 on a substrate 11.
Attempts have been made to improve the luminous efficiency of these organic EL devices. With the prior art device configuration, however, for reasons of poor hole blocking of the electron injecting and transporting layer, it was difficult to achieve efficient recombination of electrons with holes in the light emitting layer and hence, to provide a device with a fully satisfactory efficiency.
An object of the present invention is to provide an organic EL device having an excellent electron and hole injecting efficiency and improved light emitting efficiency, which can be manufactured at reduced cost and which can be operated at low drive voltage.
This and other objects are achieved by the present invention which is defined below.
(1) An organic electroluminescent device comprising a hole injecting electrode, a negative electrode, and at least one organic layer between the electrodes; wherein
at least one of said at least one organic layer has a light emitting function;
a high resistivity inorganic hole injecting layer is provided between said hole injecting electrode and said organic layer,
said high resistivity inorganic hole injecting layer capable of blocking the electrons and having hole conductive paths; and
an inorganic insulative electron injecting and transporting layer is provided between said light emitting layer and said negative layer,
said inorganic insulative electron injecting and transporting layer containing at least one oxide selected from strontium oxide, magnesium oxide, calcium oxide, lithium oxide, rubidium oxide, potassium oxide, sodium oxide and cesium oxide as its main component.
(2) The organic electroluminescent device of (1) wherein said high resistivity inorganic hole injecting layer has a resistivity of 1 to 1xc3x971011 xcexa9xc2x7cm.
(3) The organic electroluminescent device of (1) wherein said high resistivity inorganic hole injecting layer contains at least one member selected from a metal and/or its oxide, carbide, nitride, silicate and boride.
(4) The organic electroluminescent device of (1) wherein said high resistivity inorganic hole injecting layer comprises oxide of silicon and/or germanium as its main component, said main component having a composition represented by:
(Si1xe2x88x92xGex)Oy
wherein 0xe2x89xa6xxe2x89xa61 and 1.7xe2x89xa6yxe2x89xa62.2, and said high resistivity inorganic hole injecting layer further comprises a metal having a work function of at least 4.5 eV or oxide thereof.
(5) The organic electroluminescent device of (4) wherein said metal is at least one member selected from Au, Cu, Fe, Ni, Ru, Sn, Cr, Ir, Nb, Pt, W, Mo, Ta, Pd and Co.
(6) The organic electroluminescent device of (4) wherein content of said metal and/or said metal oxide is in the range of 0.2 to 40 mol %.
(7) The organic electroluminescent device of (1) wherein said high resistivity inorganic hole injecting layer has a thickness of 1 to 100 nm.
(8) The organic electroluminescent device of (1) wherein said inorganic insulative electron injecting and transporting layer contains
80 to 99 mol % of the main component, and
1 to 20 mol % of the stabilizer, based on the entire components.
(9) The organic electroluminescent device of (1) wherein said inorganic insulative electron injecting and transporting layer has a thickness of 0.1 to 2 nm.